1. Field of the Invention
The present invention relates to, for instance, a terminal device, such as a personal computer, cellular phone, a personal digital assistant (PDA) and a portable audio player, and more specifically to a device equipped with a function of recording/playing back of content.
2. Description of the Related Art
In recent years, a growing number of terminal devices, such as a notebook-sized personal computer and a cellular phone equipped with a function of playing back content which has been received via communication network or recorded on a recording medium are deployed in the market. With using such devices, a user may play back content including favorite tunes and videos to enjoy them anywhere, and it is very convenient for the user.
Meanwhile, recently, to protect copyright, content with playback validity period and a playback expiration date set thereto have been increasingly deployed for a while. For playing back such types of content, a conventional information terminal device compares the playback validity period and the playback expiration date set to the content with a current time clocked by a built-in clock. The conventional terminal device is so constituted as to execute a content playing back operation, if the current time is within the playback validity period or the playback expiration date.
However, in general, the built-in clock in the terminal device has a clocking error depending on a self-running clock, i.e. drift of timing. Therefore, a terminal device having a built-in clock with a large clocking error cannot accurately manage the playback time limit of the content. The built-in clock installed in such type of terminal device generally has a function of a time adjustment by hand so that the user can freely adjust its clocking time. Accordingly, if the user changes the clocking time of the built-in clock backwards on purpose, even when the validity period expires, the terminal device becomes possible to play back the content even when the content is out of the expiration date, and as a result, the terminal device cannot prevent illegal use of the content.
Therefore, a terminal device employing a secure clock, as a built-in clock, having a function to receive accurate standard time data from a cellular phone base station and adjust the clocking time on the basis of the standard time has been proposed. With using this type of terminal device, the user may determine the playback time limit of the content on the basis of the clocking time tuned to the standard time data. So that, it becomes possible to accurately determine the playback time limit by eliminating influence of the clocking error caused by the built-in clock. And also it becomes possible to prevent the illegal use of the content by eliminating influence of an intentional change in clocking time by the user (for instance, Jpn. Pat. Appln. KOKAI Publication No. 2003-169376).
On the other hand, in a CPRM for SD-video 0.94 specifications that are storage specifications for movie video content in a memory card, new specifications of the “Time Usage Rule” not using the secure clock have been employed. This specification set a timestamp file common to content in a secured area of the memory card and updates the time of the timestamp file to the clocking time by the built-in clock at playback start time and playback completion time of the content, respectively. In playing back the content, the terminal device compares the clocking time of the built-in clock with the time of the timestamp file and determines that the clocking time of the built-in clock is possible to be altered if the time of the built-in clock is older than that of the timestamp file then does not play back the content (for example, refer to http://www.4centity.com/ “CPRM for SD-Video 0.94 specifications”).
However, in the aforementioned each proposal, there are problems described below. That is, a proposal disclosed in the foregoing Jpn. Pat. Appln. KOKAI Publication No. 2003-169376 cannot be employed to a terminal device not having the secure clock therein. In contrast, the latter specifications disclosed in http://www.4centity.com/ “CPRM for SD-Video 0.94 specifications” is applicable to the device not having the secure clock. But the terminal device not having the secure clock has to perform time update processing of the timestamp file at every playback of the content. In general, in playing back the content, the terminal device ought to perform only reading-out processing of the content from the memory card. However, in the playback start time and playback completing time of the content, writing-in processing of data to the memory card is required, respectively, so that the terminal device has a problem such that a load of playback start processing and playback completion processing on the terminal device becomes heavy.